Introduction 3. Metabolic and non-metabolic mechanisms to regulate the cGMP concentration in cells 3.1. Guanylate cyclase (EC 4.6. 1.2)

3.2. Cyclic nucleotide phosphodiesterase (EC 3.1. 4.17 and EC 3.1. 4.35) 3.3. Transport and buffering of cGMP 4. Targets of cGMP action 4.1. cGMP-dependent protein kinase 4.2. Cyclic nucleotide-gated ion channels 4.3. cGMP-binding phosphodiesterases 4.4. Other potential targets of cGMP action 5. Sensory transduction in vertebrate rod and cone photoreceptors 5.1. Rod and cone photoreceptor cells 5.2. Photoreceptor electrophysiology 5.3. Molecular components of visual excitation 5.4. Molecular components of photoresponse termination and adaptation 6. Regulation of cGMP levels in photoreceptors 6.1. cGMP metabolic flux sets the dynamic steady state cGMP concentration 6.2. Buffering and sequestration of cGMP in rod outer segments 7. Targets of cGMP action in photoreceptor cells 7.1. Cyclic nucleotide-dependent protein kinases 7.2. Cyclic nucleotide-gated ion channel 7.3. Photoreceptor PDE6 8. Summary